There is known a conventional transmission mechanism having a structure in which a centrifugal start-up clutch is provided on an output side (i.e. driven side) which transmits the driving force of an engine to driving wheels via a belt-type continuously variable transmission system. Since the transmission mechanism allows the interruption of torque transmission between the driving wheels and the continuously variable transmission system when a vehicle suddenly stops, the gear ratio of the continuously variable transmission system can be shifted to a low gear-ratio range (i.e. a low-ratio range) by the rotation of the engine. But at that time, a large torque from the deceleration is placed on the output side of the continuously variable transmission system. To counteract the torque, the capacity of the start-up clutch should be increased, which leads to increases in the size and weight of the system. In addition, fluctuations in torque resulting from changes in the number of revolutions of the engine are input directly to the continuously variable transmission system so that a load not less than required is undesirably imposed on a belt of the continuously variable transmission system.
In a structure in which a start-up clutch is provided on the input side (i.e. the driving side) of a continuously variable transmission system, the gear ratio of the continuously variable transmission system is less likely to return toward the low-ratio range when a vehicle suddenly stops. In such a case, if the gear ratio of the continuously variable transmission system is not in the low-ratio range at the time of restart, sufficient acceleration performance cannot be obtained.
Japanese Patent Document JP-A No. 14004/2003 discloses a variable-speed drive where a start-up clutch is provided on the input side and which opens the start-up clutch when the vehicle comes to a sudden stop to prevent an engine stall. Power transmission to the output shaft of the continuously variable transmission system is then interrupted by using a switching mechanism for disconnecting and connecting power transmission to the driving wheels to reduce the load on the belt. The drive subsequently re-engages the start-up clutch to return the gear ratio toward the low-ratio range.
However, the technology of Japanese Patent Document JP-A No. 14004/2003 has the problem that the gear ratio of the continuously variable transmission system cannot be returned to the low-ratio range in the state in which the number of revolutions of the crankshaft of the engine is reduced to a specified value or less and the rotation of the driving pulley has stopped. The technology in Japanese Patent Document JP-A No. 14004/2003 also has the problem that a clutch capable of controlling disconnection and connection becomes necessary on each of the input and output sides of the continuously variable transmission system, which leads to a complicated structure, increased weight, and an increased number of assembly stops.